1) Field of the Invention
The present invention relates to a process for producing a polycarbonate resin with high flowability, and more particularly to a process for producing a polycarbonate resin with high flowability in which both molding transcription and cycle times-shortening ability suitable for the production of CD, LD, Read Only Memory (ROM) disk, Direct Read After Write (DRAW) disk, Erasable Direct Read After Write (E-DRAW) disk, etc, are excellent and furthermore the amount of stain on a stamper is small.
2) Prior Art
Polycarbonate resin is used as molding materials for optical parts such as materials for optical recording media, lenses, fibers, etc., since it has excellent optical characteristics and furthermore low equilibrium water absorption rate, excellent heat resistance, dimensional stability and mechanical characteristics such as impact strength, etc.
Particularly, in optical recording media, a high quality of polycarbonate resin is required from the aspects of prevention against stain on stamper during production of substrate, improvement of molding transcription, durability of recording layer, long time reliability of polycarbonate resin substrate per se, etc. For example, as described in Japanese Patent Kokai (Laid-open) Nos. 63-278929, 63-316313 and 1-146926, it has been suggested to reduce low molecular weight carbonate compounds and unreacted bisphenol.
However, even in the above-mentioned prior arts, it is still unsatisfactory to prevent stain on a stamper.
Further, as a method for reducing low molecular weight carbonate compounds being formed during polycondensation reaction, Japanese Patent Kokoku (Publication after examination) No. 6-23243 discloses a method for specializing a procedure for charging raw materials and a caustic alkali concentration in polymerization reaction. Japanese Patent Kokai (Laid-open) No. 3-109420 discloses a method for reducing a production amount of dimer of molecular weight modifier by using a tubular reactor in phosgenating reaction and adding the molecular weight modifier inside the tubular reactor or in its exit.
However, in the above-mentioned methods, the content of low molecular weight carbonate compounds having the range of molecular weight 1,000 or below is not sufficiently reduced.
Japanese Patent Kokai (Laid-open) Nos. 6-336522 and 7-165899 describe the production of an aromatic polycarbonate having a low content of low molecular weight oligomers and a narrow molecular weight distribution. However, when an aromatic polycarbonate having a viscosity average molecular weight (Mv) of 13,000 to 20,000 is produced, low molecular weight carbonate compounds having the range of molecular weight 1,000 or below is not sufficiently reduced. Further the aromatic polycarbonate thus obtained has low flowability since the amount of produced oligomers to provide high flowability is very small, so that it is difficult to use it as the material for optical recording medium.
Also a method for removing low molecular weight carbonate compounds, unreacted bisphenol and remained halogenated solvent in a purifying process has been suggested. For example, the methods for extracting the above-mentioned substances with acetone as a poor solvent for polycarbonate have been suggested (Japanese Patent Kokai (Laid-open) Nos. 63-278929 and 4-306227) However, the above-mentioned methods have such drawback that deterioration of flowability of molten resin itself is caused during molding since, in comparison with a case of not conducting extraction with acetone, not only low molecular weight compounds to stain on a stamper, but also polycarbonate oligomer contributing much to flowability is removed during production of substrate. Furthermore, in the methods, a slight amount of acetone remains in a product, i.e, pellets since extraction with acetone is conducted, so that thermal degradation such as lowering of molecular weight and coloration during molding occurs.
Further, as the optical recording media, materials having high flowability during melting have been required in a disk injection molding from the viewpoints of reduction of birefringence and improvement of molding transcription and cycle times-shortening ability.
For example, as methods for improvement of flowability during melting, Japanese Patent Kokai (Laid-open) No. 60-215020 discloses that high flowability is provided by modifying terminal groups of polycarbonate to long chain alkyl groups and Japanese Patent Kokai (Laid-open) No. 61-123658 discloses that high flowability during melting is provided by adding oligomer to a polycarbonate resin. However, although improvement into materials with high flowability can be attained by modification of terminal groups or addition of oligomer, it is impossible to prevent stain on stamper in the production of substrate because low molecular weight carbonate compounds having the range of molecular weight 1,000 or below are still contained.